1. Field of the Invention
The present invention relates to a wheel steering apparatus for use in association with an active rear steering (ARS) system of an automotive vehicle or the like.
2. Description of the Prior Art
An automotive vehicle having its front wheels adapted to be steered through a steering wheel has recently come to employ an active rear steering (ARS) system. The ARS system is an electronic control system for monitoring the wheel speed of the front wheel pair of the automotive vehicle and for controlling the steering of the rear wheel angle so that the vehicle body can attain a target body slip angle. A wheel steering apparatus for the rear wheel pair that can be used in association with this system is generally so configured that a reciprocatingly movable rod drivingly connected with support members for the rear wheels can be reciprocatingly moved by an electronically controlled drive motor through a screw mechanism used to translate a rotary motion of the drive motor into a linear motion. The screw mechanism referred to above includes a nut member in the form of generally trapezoidal threads and a generally trapezoidal screw shaft defined by a portion of the rod such that the nut member in the form of the trapezoidal threads can be rotated by the drive motor to thereby move the screw shaft in a direction axially thereof.
While the trapezoidal threads can convert a rotational torque of the nut member caused by the drive motor into an axial thrust force (a positive thrust operation), the positive operating efficiency of the trapezoidal threads is generally 50% or lower and, therefore, a loss is considerably large. For this reason, it is generally considered necessary to employ a relatively large size drive motor.
As a means for substantially solving this problem, it is contemplated to substitute therefor a ball screw mechanism capable of securing a relatively high working efficiency regardless of operating directions. However, if the ball screw mechanism is used in the rear wheel steering apparatus, since the efficiency is high during a reverse operation, the steerability of the vehicle appears to be adversely affected as a result of increase of the torque generated in a rotor of the drive motor, when the rear wheel steering apparatus is affected by an external force which would act on the wheel steering apparatus when, for example, the vehicle then running is hit by crosswind.
In view of the foregoing, the present invention has its primary object to provide a wheel steering apparatus capable of exhibiting a stabilized vehicle steerability without transmitting an external disturbance torque to a motor rotor even when the wheel receives an axially acting external force.
It is another important object of the present invention to make it possible to assembly the wheel steering apparatus in a compact size and to reduce the number of assembling steps.
In order to accomplish the foregoing objects of the present invention, there is provided a wheel steering apparatus including a reciprocatingly movable rod drivingly connected with a support member for steerably supporting a wheel, and a ball screw mechanism. The ball screw mechanism includes an outer helical groove defined on at least a portion of an outer periphery of the rod, a nut member having an inner peripheral surface formed with an inner helical groove defined therein and relatively rotatably mounted on the rod with the inner helical groove aligned with the outer helical groove, and a series of balls rollingly accommodated in part within the outer helical groove and in part within the inner helical groove, which nut member is driven by a drive motor for reciprocatingly moving the rod to steer the wheel. The wheel steering apparatus also includes a reverse input limiting means provided in a rotational transmission system for transmitting rotation of the drive motor to the nut member. The reverse input limiting means referred to above is operable to prevent a rotor of the drive motor from being rotated in response to an external force acting on the wheel.
According to the present invention, since the rotation of the drive motor is transmitted to the steering rod through the ball screw mechanism, the working efficiency is high with a minimized loss and the drive motor of a compact size is sufficient. While the ball screw mechanism can exhibit a high efficiency even during a reverse drive operation, the provision of the reverse input limiting means is effective to prevent the rotor of the drive motor from being rotated by the effect of an external force which would act on the wheel steering apparatus when, for example, the vehicle then running is hit by crosswind. Accordingly, even though the wheel is affected by the axially acting external force, no external disturbance torque is transmitted to the rotor of the drive motor, thereby to exhibit a stabilized vehicle steerability.
Thus, since no external force is transmitted from the vehicle wheels to the motor rotor, a motor control system can be advantageously simplified. Also, even when the wheel steering apparatus is affected by the external force transmitted from the vehicle wheels, no retaining torque for the drive motor is necessary and, hence, the power consumption by the drive motor can be reduced. Where the vehicle wheels are steered by the utilization of the output from the drive motor, the use of the ball screw mechanism enables it to be rotated lightly and, accordingly, the drive motor can have a reduced capacity.
The reverse input limiting means may be a clutch assembly. This clutch assembly includes a rotatable input side ring and a rotatable output side ring and is operable to transmit rotation from a rotatable input side ring to a rotatable output side ring, but preventing the transmission of the rotation from the output side ring to the input side ring. By this function, even though the wheel is affected by the axially acting external force, no external disturbance torque is transmitted to the rotor of the drive motor, that is, a reverse input is limited and, accordingly it can have a stabilized vehicle steerability. This clutch assembly may be a two-way clutch assembly.
Specifically the clutch assembly may include an outer race fixed to a stationary member, in which the input side ring and the output side ring are inserted axially from opposite ends thereof to permit the outer race to rotatably support the input side ring and the output side ring, a plurality of engagement elements disposed between mutually confronting surface of the outer race and the output side ring for engaging the outer race and the output side ring together when the outer race and the output side ring undergo a relative rotation, and a retainer fixedly connected with the input side ring for displacing the engagement elements between an engaged operative position and a non-engaged position. The retainer and the output side ring are connected with each other for angular play in a direction of rotation. The magnitude of play of the retainer relative to the output side ring is preferably so chosen as to be virtually equal to, for example, the distance over which the engagement elements move from the non-engaged neutral position to the engaged operative position. The engaged operative position is where the clutch assembly is held in a locked condition.
The clutch assembly of the above described construction operates in the following manner. When the input side ring and the output side ring undergo a relative rotation with each other, the retainer fixed to the input side ring rotates relative to the output side ring an angular distance corresponding to the magnitude of play in the direction of rotation and, in response to the rotation of the retainer, the engagement elements displace towards the engaged operative position. When starting from this condition the input side ring further rotates, the input side ring and the output side ring are connected together through the retainer and, therefore, the output side ring rotates and the rotation thereof is consequently transmitted to an output side. Conversely, when starting from the above described condition the output side ring is rotated in a reverse direction by the effect of, for example, the axially acting force imposed on the wheel, the engagement elements then held at the engaged operative position are locked with the output side ring consequently coupled with the outer race. Coupling of the outer side ring with the outer race results in interlocking of the output side ring with the outer race and no rotation is therefore transmitted to the input side ring. Thus, although the rotation can be transmitted from the input side ring to the output side ring, transmission of the rotation from the output side ring to the input side ring is interrupted.
In this clutch assembly, the engagement elements may be of a type operable to engage the outer race and the input side ring with each other in two directions with respect to the direction of rotation. By this arrangement, even when the wheel is affected by the external force in any direction, no disturbance torque will be transmitted to the rotor of the motor and, hence, a further stabilized vehicle steerability can be obtained.
In the wheel steeling apparatus according to the present invention, where the reverse input limiting means is a clutch assembly, component parts of the clutch assembly and the nut member of the ball screw mechanism may be integrated together to form respective parts of one-piece component.
Integration of the component parts of the clutch assembly with the nut member of the ball screw mechanism makes it possible to achieve a structure compact in size in an axial direction with no necessity to connect the clutch assembly and the nut member together during the assemblage, resulting in reduction in the number of the assembling steps. Also, possible generation of rattling motion of the nut member of the ball screw mechanism which would result from a poor precision of connection between the component parts of the clutch assembly and the nut member of the ball screw mechanism can be relieved, thereby stabilizing the working torque.
The one-piece component referred to above may include the output side ring and the nut member of the ball screw mechanism integrated together. In such case, the clutch assembly may include an outer race fixed to a stationary member with the output side ring of the one-piece component being provided rotatably coaxially of the outer race, a plurality of engagement elements disposed between mutually confronting surface of the outer race and the output side ring for engaging the outer race and the output side ring together when the outer race and the output side ring undergo a relative rotation, a retainer fixedly connected with the input side ring for displacing the engagement elements between an engaged operative position and a non-engaged position and interlocked with the input side ring, and a connecting means for connecting the input side ring with the output side ring for angular play in a direction of rotation. Interlocking of the retainer with the input side ring may be accomplished by the use of an integral component parts in which the retainer and the input side ring are integrated together, or may be accomplished by connecting the retainer separately with the input side ring.
The clutch assembly of this design operates in the following manner. Let it be assumed that in a rotation halted condition the engagement elements are held at the engaged operative position. Starting from this condition and when the output side ring is tended to rotate, the engagement elements held at the engaged operative position causes the outer race and the output side ring to be engaged with each other, that is, to be locked with each other. The outer race is fixed to the stationary member and is hence in a fixed condition and, accordingly, the output side ring is barred from rotating due to its engagement with the outer race through the engagement elements, with no rotation consequently transmitted to the input side ring. When in this condition the input side ring is rotated, the retainer integral with this input side ring rotates, causing the engagement elements to displace towards the non-engaged position to thereby release the lock. Further rotation of the input side ring results in connection of the connecting means, then in a play condition, with the output side ring with the rotation of the input side ring consequently transmitted to the output side ring. In this way, while the rotation can be transmitted from the input side ring to the output side ring, the transmission of the rotation from the output side ring to the input side ring can be interrupted.
Even this clutch assembly of the above described construction may be a two-way clutch assembly. In such case, an outer surface of the output side shaft may be formed with a flat cam face or two directional cam face portions inclined in respective directions opposite to each other in the direction of rotation, and rollers may be interposed between the cam face portions and the outer race, each of said rollers serving as the respective engagement element.
According to this design, in the event that while the engagement elements are held at the engaged operative position of the cam faces the output side ring is tended to rotate, rotation in one direction is locked by engagement of the rollers at the cam face portions in such one direction while rotation in the other direction is locked by the cam face portions in such other direction. Accordingly, rotation in these two directions counter to each other is interrupted. On the other hand, in the event of rotation of the input side ring, regardless of whether it rotate in any of the opposite directions, the retainer causes the engagement elements towards the non-engaged position to thereby release the lock with the connecting means consequently held in position to connect with the output side ring to thereby transmit the rotation and, accordingly, transmission of the rotation to the output side ring is possible. In this way, even though the wheel is affected by an external force in any direction, no disturbance torque is transmitted to the rotor of the drive motor and, accordingly it can have a stabilized vehicle steerability.